Applied Surface Science 256 (2009) 213–216

Optical properties of Cd1�xZnxTe thin films fabricated through sputtering ofcompound semiconductors

Pushan Banerjee a,*, Rajiv Ganguly b, Biswajit Ghosh a

a Advanced Materials and Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Kolkata 700032, Indiab Institute of Engg. & Management, Saltlake Electronic Complex, Kolkata 700091, India

A R T I C L E I N F O

Article history:

Received 20 May 2009

Received in revised form 20 July 2009

Accepted 31 July 2009

Available online 8 August 2009

PACS:

78.67.Pt

81.15.Cd

78.55.�m

Keywords:

CZT

Multilayer

Sputtering

Photoluminescence

Photoresponse

A B S T R A C T

Cd1�xZnxTe thin film fabrication is necessary for its photovoltaic and imaging applications in large scale.

Thermally annealed and thereby interdiffused r.f. sputtered multilayers comprising of CdTe and ZnTe

have been utilized here for the fabrication of Cd1�xZnxTe thin films. Photoluminescence and change of

resistance of the multilayer under illumination were studied using different annealing temperatures and

varying number of repetitions. It was found that three number of repetitions annealed at 300 8Cexhibited the best results.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Cadmium based binary and ternary II–IV compound semi-conductors have attracted considerable interest because of theirwide applications in optoelectronic devices [1]. The propertiesof Cd1�xZnxTe (CZT) make it a prime candidate for terrestrialphotovoltaic applications [2,3]. This ternary structure is one of theimportant semiconductor compounds, as a top device in a highefficiency tandem solar cell structure, due to its tunable physicalparameter [4]. CZT is a ternary semiconductor with a tunablebandgap of 1.44–2.26 eV. Solar cells with efficiencies of 20% orhigher can be achieved by using a tandem solar cell structure,which consists of a top cell and a bottom cell connected in series. Inthis kind of a structure, the light passes through the top cell firstwhich has a wider bandgap to absorb light of shorter wavelengthsand the remaining light passes through the bottom cell with alower bandgap to absorb light of higher wavelengths. For a tandemcell structure, the ideal bandgap for the top and bottom cell is 1.7and 1 eV, respectively. Cadmium Zinc Telluride is a suitablecandidate for the top cell and copper indium gallium diselenide

* Corresponding author. Tel.: +91 3324146823; fax: +91 3324146853.

E-mail address: b_pushan@rediffmail.com (P. Banerjee).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.07.112

(CIGS) with a bandgap of around 1 eV is a suitable candidate forthe bottom cell. Also, the high atomic number, tunable energyband gap to minimize leakage currents at room temperature, highdetective quantum efficiency, good charge transport, highresistivity and high intrinsic mobility-lifetime (mt) products forelectrons and holes offer it an attractive candidature as a goodradiation detector and imaging device material.

For production of large area applications of CZT basedphotovoltaic devices, thin film technology has to be applied.Several methods have been used to prepare CdZnTe films, such thatmolecular beam epitaxy [5], liquid phase epitaxy [6], electro-deposition [7], laser ablation [8], thermal evaporation [9],sputtering [10], and metal-organic chemical vapor deposition(MOCVD) [11]. Multilayer method of deposition was also triedby diffusion of elemental Zn into CdTe [12]. From this point ofview, the present work describes the fabrication of CZT thin filmsusing interdiffusion of sputtered layers of CdTe and ZnTe andtheir subsequent analysis through photoluminescence andphotoresponse.

2. Experimental methods

At first, r.f. sputtered films of CdTe and ZnTe were deposited onglass slides at a pressure of 0.1 mbar with a net power of 200 W

Table 1Required thickness of individual layers for sputtering.

No. of repetitions Thickness (nm) of

CdTe ZnTe

1 510 280

2 255 140

3 170 93.3

4 127.5 70

Fig. 1. (a) PL spectra for multilayer annealed at 200 8C. (b) PL spectra for multilayer

annealed at 250 8C. (c) PL spectra for multilayer annealed at 300 8C.

P. Banerjee et al. / Applied Surface Science 256 (2009) 213–216214

using 4 in. diameter targets of CdTe and ZnTe. CdTe was thebottommost layer while ZnTe was kept as the topmost layer, so asto ease the future electrical contact fabrication to the structure. Thenumber of CdTe–ZnTe sequence was varied from one to four andthe corresponding thicknesses are given in Table 1.

The sputtered films were annealed in the same level of vacuumat 200, 250 and 300 8C—only mixing among the telluride layerswere required in the present case to form the alloy. Also it wasfound that for sputtered films, annealing above 300 8C resulted indevelopment of pinholes on the film or their detachment from thesubstrate by breaking. The annealed structures were then testedusing room temperature photoluminescence (Perkin-Elmer LS-55spectrometer) property through an excitation at 490 nm. Thechange of resistance of the multilayer with time under illumina-tion (photoresponse) was recorded using vacuum evaporatedsilver contacts on the surface.

3. Results and discussion

3.1. Results of photoluminescence

The PL spectra are shown in Fig. 1a–c, where ‘‘L’’ in the legenddenotes the number of repetitions. In the photoluminescencespectra recorded for all the multilayer thin films, the generalpattern is the presence of two broad peaks for each sample—alower intensity peak centered about 662 nm (1.87 eV) and anotherhigher intensity at around 740 nm (1.67 eV). The intensities ofthe two peaks changed for each sample whereas the energeticpositions are the same. This behaviour can be explained from theproperty of tellurium clusters as an iso-electronic exciton trap inII–VI compounds, as reported earlier by several authors. Roessler[13] found in CdS:Te that for concentration of Te around 1018/cm3,the PL spectra (with excitation of 435 nm) showed a higher energyband around 600 nm. As the Te doping was raised gradually, lowerenergy band appeared and eventually dominated the spectrumfor Te concentrations above 1020/cm3. The high-energy band wasattributed to an exciton trapped at an isolated Te atom on a sulfursite, whereas the lower energy band resulted from trapping by twoTe atoms on nearest neighbour sulfur sites. Cuthbert and Thomas[14] also showed that in case of CdS1�xTex the spectrum with lowtellurium concentrations was due to radiative recombination of anexciton bound to a Te atom. Similarly, tellurium was found as iso-electronic exciton traps in several other II–VI compounds, such asCdSe [15], ZnSe1�xTex [16] as well as in ZnSe–ZnTe superlattice[17] and CdTe/CdS combination [18].

From the discussions made by the above authors, it can beinferred here that the higher energy band around 662 nm possiblyoriginated from a bound hole and electron recombining at atellurium atom. The 740 nm band corresponds to the radiativedecay of a hole and electron bound to two Te atoms at nearestneighbour sites. In analogy to the cases of CdS:Te and CdS/CdTe(where tellurium was forming a mixture with CdS), it can beinferred that for high concentration of tellurium, the height of thepeak around 740 nm is indicative of interdiffusion among thelayers.

Thus for sputtered multilayers, PL peak of at 662 nm show thatexcitonic transition related to single tellurium atom is highest withtwo no. of repetitions corresponding to annealing temperatures of200 and 250 8C. For three repetitions both peaks are most intense

Fig. 2. (a) Photoresponse of multilayer annealed at 200 8C. (b) Photoresponse of

multilayer annealed at 250 8C. (c) Photoresponse of multilayer annealed at 300 8C.

P. Banerjee et al. / Applied Surface Science 256 (2009) 213–216 215

when annealed at 300 8C. Also, the intensity of the 740 nm peakis highest at 300 8C among the three annealing temperatures,indicating thereby that the interdiffusion (and correspondingdefect generation) is maximum at that temperature.

3.2. Results of photoresponse

The multilayers were kept under a tungsten halogen lampwith 100 mW/cm2 intensity and the changes in their resistance(measured across two silver contacts placed 1 cm apart over thefilm surface) were recorded as a function time of illumination upto3 min. The dark resistances were taken as that at time ‘‘0’’. Theindividual resistances being different for each film, the normalizedvalues of the resistances were plotted by taking the dark resistanceas unity, so as to provide a convenient way of observing thefractional change in resistance under illumination.

Fig. 2a–c shows the fall in resistance (measured across the twosilver contacts) with time at room temperature for the multilayers.When illumination is applied on the film, the initial high value ofthe photocurrent was due to the absorption of photon by the film.This created pairs of free holes in valence band and free electrons inconduction band. Most of the electrons were from the surface ofthe multilayer that moved from valence band to the conductionband. It increased the process of pair generation initially, which inturn raised the carrier concentration, resulting in lowering ofresistance. The measured resistance fell with time and after sometime either it showed a tendency of saturation or the rate of changewas very small. The decreased rate of generation of carriersdecreased with time and the process of recombination taking placeto decrease the value of photocurrent resulted in a situation wherethe process of generation of charge carrier and recombinationreached an equilibrium under constant illumination. As a result,the ultimate pattern of change in resistance with time was almostflat.

From the pattern of photoresponse it was found that (i) forannealing at 200 8C, four repetitions had the lowest response whilethree repetitions showed highest response. (ii) For annealingat 250 8C, single repetition showed only 10% fall while threerepetitions exhibited about 50% change in resistance underillumination. (iii) Annealing at 300 8C resulted in fall in resistanceby 45% for three repetitions—the decrease in resistance for otherrepetitions being much less. Thus, the multilayer with threerepetitions showed the best response to light at all annealingtemperatures.

4. Conclusion

CZT thin films have been prepared using multilayer comprisingof r.f. sputtered CdTe and ZnTe. The structure was annealed atvarious temperatures with upto four numbers of repetitions ofCdTe–ZnTe sequence. Results of photoluminescence and photo-response showed that multilayer annealed at 300 8C with threerepetitions of CdTe–ZnTe showed best behaviour. Further works inthis area are on the way to improve the performance of the film andits applications in photovoltaic and imaging devices.

Acknowledgements

One of the authors (Pushan Banerjee) gratefully acknowledgesthe support provided by Council of Scientific and IndustrialResearch, Govt. of India for carrying out this research. The helpextended by Dr. Abhijit Saha of UGC-DAE CSR, Kolkata center,towards taking PL measurements is also thankfully acknowledged.

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